The present invention relates generally to apparatus for measuring characteristics of a circuit element such as a transistor, a field effect transistor (FET), etc., and in particular to apparatus for measuring interdependent characteristics of such a circuit element, such as the threshold voltage of an FET.
Conventionally, devices called SMUs have been used to measure parameters of the samples of a semiconductor, such as a transistor, a field effect transistor (refer to Japanse Patent: Tokkaisho 58-148506). An SMU is capable of supplying a regulated DC voltage to a sample while measuring the DC current supplied to the sample, or supplying a regulated DC current to a sample while measuring the voltage applied to the sample. In cases where the characteristics of the semiconductor sample can be measured by using a conventional measuring appratus including a plurality of SMUs which are independent from each other, the set up and measurement can be effectively carried out in one step with the SMUs thereafter operating independently from one another.
However, there are some cases in which the characteristics cannot be measured with independent SMUs. Measuring the DC current amplification factor h.sub.FE of a transistor, or the threshold voltage V.sub.TH of an FET requires a coodinated operation of multiple SMUs. To make the V.sub.TH measurement, the voltage applied by the SMU connected to the gate terminal of the FET must be varied until the voltage and the current measured by the SMU connected to the drain terminal of the FET are at the predetermined values at which the V.sub.TH measurement is to be made.
One prior art method for measuring characteristics involving interdependent circuit parameters is shown in FIG. 4. The measurement circuit in FIG. 4 comprises SMU 401, a summing amplifier 402, an integrator 403, and a sample FET 404 connected to form an analog feedback loop circuit. In FIG. 4, the loop is operated such that a difference signal e.sub.o between a voltage signal V.sub.M representing the drain current I.sub.D and a reference voltage signal V.sub.MREF is integrated by the integrator 403 to produce a negative feedback signal to the gate of the FET 404. Accordingly, the loop converges to make V.sub.M =V.sub.MREF, the threshold voltage V.sub.TH can be measured at a predetermined drain current value if the signal V.sub.MREF is set to the proper value V.sub.M which corresponds to the drain current I.sub.D at which the measurement is to be made.
In order to assure the stability of the negative feedback loop, however, it is necessary to make the integration constant T.sub.k of the integrator 403 sufficiently large so that the total gain becomes lower than 1 at frequencies in which phase delays, comprised of the response delay of the FET 404, the delay in conversion of the drain current into an output voltage of the SMU 401 and the like exceed 90 degrees. If the integration constant T.sub.k is set smaller, positive feedback may be produced, causing the loop circuit to oscillate making measurement impossible. This requirement limits the slew rate for the gate voltage, and consequently limits the search rate. Typical search rates are on the order of 100 ms/V. Another disadvantage of this method is that the operator needs a high degree of knowledge about setting the measuring conditions in accordance with characteristics of a sample to make a proper measurement. The result is that the operation of the measurement instrument becomes extremely complicated.
An object of the present invention is to provide an apparatus for measuring characteristics of a circuit element using a plurality of SMUs in coordination, and setting the measurement conditions using a two mode technique with a fast search mode followed by an analog feedback mode, to make it possible to perform highly stabilized measurements at a high speed and to eliminate successive changes in setting in the measuring conditions.
The circuit element characteristics measuring apparatus of the preferred embodiment of the present invention applies a test input signal from each of a plurality of SMUs to the terminals of a circuit element under test and measures an output signal produced by the circuit element which is a function of one or more of the input signals. The SMUs are supplied with initial values of the applied signals on the basis of the measuring conditions and known characteristics of the circuit element to be measured. Then, a two mode approximation procedure is used with a fast search mode to make the dependent measured signal approximately equal to a reference value, followed by a feedback mode to stabilize the dependent measured signal at the reference value while the measurement is made. In the search mode, one of the applied signals is rapidly changed in one direction while monitoring the dependent measured signal. Because no feedback loop is involved, a very fast slew rate can be used, on the order of 2 ms/V. Then, in the feedback mode, a negative feedback loop is switched in to superimpose the integrated difference between the output signal and the reference value on the signal applied by the SMU to make the output signal coincide with the reference value.
Accordingly, the signal values applied by the SMUs need to be set only once for each type of device tested, so that high speed set up and testing can be achieved.
Further, the measurement can be made very quickly because of the fast response of the search mode. The negative feedback loop can be set with parameters calculated in advance at a relatively conservative response, so it never becomes unstable, without appreciably affecting the speed of the measurement.